Tooth preparation guides are a crucial tool to ensure adequate tooth reduction for achieving success and longevity of the final restoration. These guides may also be referred to as reduction guides or tooth preparation indexes.
Reduction guides may be utilized and fabricated for use with all teeth and all restoration designs. That is, whether the restoration to be fabricated is a full zirconia crown, an e-max Veneer, full gold crown or a ceramic onlay, a preparation guide will provide guidance as to the amount of tooth structure reduced relative to the final restorative material selected.
Predictability for material strength under function is enhanced with the proper thickness and contours achieved with ideal tooth preparation. Additionally, restoration shade and appearance are controlled with adequate tooth reduction.
Recently, I wrote an article regarding the use of silicone tooth preparation guides. The article was entitled “The Guidebook of Preparation Design.” In that article, I referenced the use of preparation indexes to aid in determining the amount of tooth reduction. I discussed the fabrication of silicone guides and various types of guides were outlined.
The examples referenced in that article included both anterior and posterior indexes for crowns, veneers and onlays. These examples provided guidance in situations where one to many teeth were prepared. But in all cases presented, the index could be stabilized by adjacent teeth so as to properly evaluate adequate tooth reduction of the prepared tooth. The non-prepared teeth provide support for the index, as well as a “visual tool” to evaluate reduction.
The guidebook article referenced a few types of reduction guides. Each guide has its specific use in different situations. The different guides provide opportunities to evaluate tooth preparation from different views. I hope you will reference that article as a means to aid your restoration design and outcome.
One of the most important and versatile guides that were referenced is the vertical index or guide. For this guide, the silicone index is sectioned vertically from buccal to lingual, generally through the mid-buccal to mid-lingual aspect of the prepared tooth. The sectioned guide is then placed on the patient's non-prepared supporting teeth, and reduction may be visualized directly.
For example, if a lower first premolar is to be prepared for an e-max full coverage restoration, the silicone index is fabricated on the diagnostic wax-up of the tooth. The proposed contour changes are incorporated into the wax-up and are then indexed into the silicone matrix.
Once the tooth is prepared, the vertical index is fabricated by sectioning the mid-buccal to mid-lingual silicone. The distal aspect of the silicone is then placed over the first and second molars. The tooth reduction may then be visualized directly. Using a periodontal probe, the amount of tooth reduction is measured. This is a very precise method of correlating tooth preparation to the material space requirement of the proposed final restoration. This type of reduction guide may be utilized in any tooth position and for any type of proposed restoration.
But, at times, tooth preparation is required of the most posterior tooth in an arch. Alternatively, the entire quadrant is being prepared and restored. In these circumstances, supporting teeth for reduction guide stabilization may not be available. The vertical guide cannot be used, intra-orally, in these situations.
It is not advised to “just guess” about the tooth reduction of the most posterior tooth. But, there are a couple of techniques that may be utilized to more precisely determine the amount of tooth structure removed to help insure adequate strength and shade of materials.
There are two reduction guide techniques that I utilize in these more difficult, un-supported situations. Both techniques begin with the fabrication of a diagnostic wax-up to properly design the contours of the final proposed restoration. This is always the starting point for restoration design and fabrication.
Fabricating the provisional restoration
The first technique involves fabricating the provisional restoration. If a distal tooth must be prepared for restoration, the diagnostic wax-up is designed and the provisional matrix fabricated from the wax-up. The matrix may be silicone, Copyplast (Great Lakes Orthodontics), or another material of choice. Once the tooth is prepared to the “assumed” depth, the provisional restoration is fabricated, either directly or indirectly to the desired contours.
Once the provisional is checked for occlusion and function, a measuring caliper may be utilized to determine provisional thickness at various points. These calipers are very accurate, to within 0.1 mm. Although this technique can provide the necessary information regarding tooth preparation depth, it requires complete fabrication of the provisional in order to gain the required information.
If inadequate preparation depth is discovered, the tooth must be re-prepared to achieve adequate material thickness. Following this re-preparation, the provisional must be re-lined, occlusion checked and re-measured to verify proper thickness. If all is well, the final impression may be taken and provisional cemented. Although this is a very accurate technique, in can be cumbersome and time consuming.
Preparation using Mach Slow or Mach II silicone
An alternative technique is one that does not require initial provisional fabrication. Again, in this technique, the diagnostic wax-up must be fabricated, along with the silicone preparation guide prior to tooth reduction. Once the tooth preparation is complete, to the best of the dentist’s skill, a quadrant alginate impression of the tooth preparation is obtained.
Once removed from the patient, Mach Slow or Mach II silicone is injected into the impression. This silicone material sets very hard (and very quickly with Mach II) within a few minutes. The silicone model is separated from the impression material (do not use silicone impression material like impregum, as the “like materials” will not separate).
The silicone reduction guide may then be vertically sectioned and be tried on the silicone model. Various slices may be utilized to evaluate various aspects of the prepared teeth. For example, the distal aspect of the preparation may be evaluated by sectioning through the distal cusps from buccal to lingual aspect of the silicone.
Subsequently, the mesial reduction may be evaluated by further sectioning the index through the mesial cusps. The Mach material is very rigid and accurate. The reduction index will be very stable for proper evaluation. A periodontal probe may, again, be utilized to verify tooth reduction.
If adequate tooth reduction has been accomplished, the provisional can then be fabricated. An additional benefit of this reduction guide technique is that the provisional may be fabricated from this Mach Slow or Mach II silicone model. Fabricating provisionals indirectly through this method is a very efficient and accurate technique!
Tooth reduction guides are invaluable tools for ensuring proper tooth reduction and providing adequate material space for final restorative materials. Tooth preparation and visualization of posterior teeth is a challenging endeavor. These reduction guides and techniques provide efficiency, accuracy, and confidence for achieving predictable and long-lasting restorations.